The invention relates to high frequency compression drivers. Compression driver is an electro-mechano-acoustical transducer of electrodynamic type that converts electrical audio signal into an acoustical signal.
Electro-dynamic transducers that turn an electrical signal into radiated acoustical sound waves are well known. Such devices are generally broken down into two categories: direct radiating electro-dynamic loudspeakers, which directly radiate the generated sound waves into open air, and indirect radiators (consisting of horns and horn drivers, that are also called compression drivers), which require additional elements such as compression chamber, a phasing plug and a horn.
In a direct-radiating loudspeaker, the diaphragm, which is driven by the voice coil, vibrates and excites the particles of the surrounding air to generate the sound waves related to the input electrical signal. Low efficiency of direct radiating loudspeakers as well as the lack of controlled directivity of radiated acoustic energy make them impractical for use in sound systems requiring high sound pressure levels and controlled directivity.
Generally, compression drivers can generate much higher sound pressure levels when compared with direct radiators and are used, predominantly in sound reinforcement and in public address systems, where the loud sound signals are of essence.
In horn loudspeakers, such as compression drivers, the diaphragm moves against a surface closely spaced thereto and generates high-pressure acoustical waves which are passed through a phasing plug to a horn. Phasing plug is essentially an acoustical adapter that connects the air volume in front of the diaphragm (called compression chamber) to the input (throat) of the horn. Phasing plug has one or several inlets with overall area smaller than that of the diaphragm. Smaller area of the inlets of the phasing plug provides air compression and the increase of the sound pressure in the compression chamber therefore increasing efficiency of the transformation of mechanical energy of the moving diaphragm into the acoustical energy of a sound signal. The phasing plug is also used to reduce the volume of air to be compressed by the vibrating diaphragm to decrease the parasitic compliance of the air in compression chamber to prevent attenuation of high frequency signals. The phasing plug is also used to cancel high frequency standing waves in air chamber through carefully positioned passageways or holes through the phase plug, and it also used to eliminate certain interfering cancellations in the generated sound waves.
The phasing plug conveys the sound signal into the horn and is essentially the beginning of the horn. The horn provides transformation of a high sound pressure level signal at the throat into a lower sound pressure signal at the mouth of the horn. The horn with a phasing plug at its beginning is essentially an acoustical transformer which matches high mechanical impedance of the vibrating diaphragm to the low impedance of open air.
A horn speaker introduces distortions at high output levels which are perceived by a listener as a lack of quality and clarity of sound. The distortions of a horn speaker are caused by several reasons. Distortion may occur due to the high and non-symmetrical mechanical stiffness of the suspension of the diaphragm. This distortion is dependent on the amplitude of the excursion of the diaphragm. Since the amplitude of the excursion increases at lower part of the frequency range of the driver, the level of this distortion also increases at low frequencies. Great deal of distortion is generated in the compression chamber because of the non-linear nature of the compression of air. Strictly speaking, there are two air chambers in a compression driver. The chamber in front of the diaphragm, namely compression chamber, is open into the horn through the orifices in the phasing plug. The chamber behind the diaphragm, called rear chamber or back chamber, is usually sealed. In spite of the similar basic nature of the air compression-related distortion in front and rear chambers, its behavior is different. The air trapped in the back chamber acts merely as a non-linear spring, somewhat similar to the non-symmetrical mechanical suspension of the diaphragm. The air in the front chamber is also non-linearly compressed during the operation of the driver, but since the front compression chamber is open into the horn, the process of compression is more complicated and so is the behavior of the corresponding distortion.
In order to understand the non-linear behavior of air enclosed in a chamber, one may consider that the diaphragm acts as a piston, reciprocating in a cylinder, which is either closed, which is typical for the rear chamber, or has an orifice of an area which is equal to the entrance of the phasing plug (this holds true for the front chamber). For adiabatic change of pressure which occurs in the cylinder, which is a compression chamber, the relationship between the total pressure and volume in the cylinder is expressed by the Boyle's law, (P.sub.0 +P(t)) (V.sub.0 -V(t)).sup..gamma. =P.sub.0 V.sub.0 =const , where P.sub.0 is atmospheric pressure, V.sub.0 is the initial volume, P(t) is the instantaneous change of the pressure in the cylinder, V(t) is the change of the volume of the cylinder, and .gamma.=1.4 is the ratio of the specific heat of the air at constant pressure to the specific heat at constant volume. As the cylinder reciprocates with equal displacement on either side of the initial reference position, the minimum and maximum values of the displacement and correspondingly, the volume, cause non-equal changes of pressure around its initial value P.sub.0. The positive change of the pressure (this corresponds to decrease of the volume) has higher amplitude than the negative change of the pressure, which corresponds to the increase of the volume. For a sealed cylinder, the volume V is expressed as V(t)=X.sub.d (t)S.sub.d where X.sub.d is the displacement of the cylinder (diaphragm), S.sub.d is the area of the cylinder (diaphragm). For the partly open cylinder, the front chamber, the change of the volume is expressed as V(t)=X.sub.d (t)S.sub.d -X.sub.t (t)S.sub.t where X.sub.t is the displacement of the air particles at the orifice of the cylinder at the entrance of the phasing plug and S.sub.t is the area of the orifice. The air in the front chamber is partly compressed and partly displaced into the entrance of the phasing plug to propagate down the horn to be radiated from the mouth of the horn. Input acoustical impedance of the horn with the phasing plug being at the beginning of the horn is frequency-dependent. It is essentially zero at low frequencies, and then it grows with frequency and reaches the constant value ##EQU1##
where .rho. is the air density, c is the speed of sound, and S.sub.t is the area of the entrances in the phasing plug. At low frequencies the compression chamber is practically open, there is no air compression and no air-related distortion occurs. At higher frequencies the impedance increases and the chamber gets "closed" (not completely though), and the pressure inside the chamber increases. As the compression of the air increases the distortion grows. Therefore, the distortion increases with frequency until the impedance of the horn reaches its maximum constant value. Obviously, the distortion also grows with the increase of pressure in the chamber. The smaller area of the entrances in the phasing plug causes higher pressure in the chamber, and correspondingly, higher level of air compression-related distortion. To decrease the level of air compression distortion in the rear chamber, its volume should be large as compared to the displacement volume of the diaphragm. Opening in the back chamber decreases the pressure in the back chamber and, correspondingly, decreases the level of distortion. Rear chamber can be opened into the cavity underneath the top plate, between the magnet and the pole piece.
The level of air compression distortion in the front chamber is a compromise with the efficiency of the compression driver as well as the level of high frequency signal. The distortion can be minimized by increasing the volume of the front chamber or the area of the openings of the phasing plug. However, the increase of volume always brings the level of high frequency signal down, and the increase of the area of the openings of the phasing plug may decrease the efficiency of the driver.
While a phasing plug is generally essential to the efficiency of a compression driver, a phasing plug is the direct cause of several problems in compression drivers. Since several paths of different length may extend from the outer periphery of the diaphragm to the horn throat, (this is typical for phasing plug placed over the convex surface of a dome diaphragm) by way of the phasing plug, the generated sound wave at the throat of the horn may be distorted due to the phasing problems. Cancellation of acoustical signal at certain frequencies may occur. In addition, since the phasing plug must be located close to the diaphragm (in order to minimize the volume of air in compression chamber) excursions of the diaphragm are limited and reproduction of low frequency signal is compromised because the displacement of the diaphragm increases at low frequencies.
Finally, the upper frequency range of typical compression devices is limited to about 14-16kHz. The limitation is explained by the inertia due to the mass of the moving diaphragm, by the increase of impedance of the inductance of the voice-coil, by the parasitic compliance of the air in compression chamber and by the occurrence of high frequency acoustic resonances in the compression chamber that cause notches on the frequency response. It is desirable that the path lengths from all portions of the diaphragm to the throat of the horn be equal to produce sound waves of the same phase at the throat. To prevent this cancellation of high frequency signal at output the differences in path lengths from the diaphragm to the throat of the horn should not exceed a quarter wavelength of the highest frequency of the signal.
In all compression drivers the dome is attached to a mounting ring or base via a compliant material known as surround of the diaphragm. The surround allows the dome to move up and down in response to the electrical signal fed to the voice coil and centers the dome both vertically and horizontally.
An important aspect of the performance of the diaphragm at high frequency is the mechanical high frequency resonances of the dome which occur well above the low frequency fundamental resonance of the mass of the diaphragm and compliance of the surround. If the diaphragm is driven at the high frequency resonances, it will produce a greater output than it will if it is driven at a somewhat higher or a somewhat lower frequency. Therefore the high frequency mechanical resonances of the diaphragm can be utilized to partially offset the mass-induced high frequency roll-off and thereby extend the useful range of a compression driver.
Resonance frequencies are dependent upon the physical properties of the material of the diaphragm and curvature of the dome. These frequencies can be estimated from the properties of the material and the curvature and length of the spherical section. Some of the materials used for construction of dome diaphragm for high frequency compression drivers include aluminum, beryllium, and titanium. Heat-treatable aluminum is a reasonable compromise for the dome diaphragm, since it is light-weight, relatively stiff, has a high fatigue strength, and has a high damping tendency that turns part of the unavoidable distortion of the moving diaphragm into heat, rather than into distorted sound.
The requirement for high frequency response coupled with high power handling presents a formidable challenge for loudspeaker designers. High frequency performance requires light, low mass diaphragm and voice coils. High power handling capacity is better provided by substantial coils and diaphragms which because of their high mass are inefficient at higher frequencies. The mid to high frequency range is usually divided into two bands and covered by two physically different driver units. The lower end (mid-range) is serviced by drivers with relatively heavy diaphragm assemblies. The high end is covered by drivers equipped with light diaphragms and small diameter coils. Several smaller drivers are required to match the output of each large mid-range unit. The solution is reliable, but not altogether satisfactory because of the obvious penalties in cost, size and weight.
Therefore, the design of wide frequency band compression drivers having high efficiency, smooth frequency response and high power handling capacity is a complicated and compromised problem. Effective reproduction of high frequency signals needs light moving assembly, very small height of compression chamber and low inductance voice coil. These requirements call into question the ability of the compression driver to reproduce lower part of the mid-band frequencies, its power handling capacity, its lower distortion. That is why the prior art is characterized by the wide variety of technical solutions to improve parts of compression drivers such as phasing plug, surround, diaphragm, and magnet assembly.
U.S. Pat. No. 3,665,124 teaches a loudspeaker which includes an annular diaphragm including a vibrating portion having an arcuate shape in cross section, such as a shape of a fraction of a circle or an ellipse, and inner and outer peripheral support portions, voice coils attached to borders between the vibrating portion and the inner and outer support portions of the annular diaphragm and the magnetic circuit which has concentric gaps for receiving the voice coils, respectively, to drive the annular diaphragm in phase with the voice coils.
In FIG. 1 of U.S. Pat. No. 3,665,124 a horn loudspeaker includes an annular diaphragm supported at its inner and outer peripheries by a frame, a voice coil attached to the diaphragm, a magnetic circuit for driving the voice coil, a diaphragm cover and an equalizer. The same construction can be used in a direct radiating loudspeaker which has a larger diaphragm. In the horn loudspeaker the borders between the support or edge portions and the vibrating portion of the diaphragm are not driven, so that the vibration of the support or edge portions effects the vibration of the vibrating portion of the diaphragm. If the vibration of the support portions acts on the vibrating portion in opposite phase, there may be caused deep dips in the frequency characteristics of the loudspeaker. The peripheral part of the diaphragm is weak since it is supported through the soft support portion, so that the diaphragm is liable to produce free vibration, resulting in turbulence in the frequency characteristics. A light and rigid diaphragm can be obtained, since the vibrating portion of the diaphragm has increased rigidity owing to the arcuate shape in cross section. The vibrating portion of the diaphragm is effectively separated from the support portions by the border driven by the voice coils, so that the vibration of one of them has minimum effect on the other. Accordingly a relatively smooth frequency characteristic can be obtained, without turbulence or distortion owing to the influence of the support portions. The vibrating area can be increased, compared with the conventional dome loudspeaker. A light and strong diaphragm can be obtained with relatively large vibrating area. Accordingly, the efficiency can be also increased. The frequency range of the piston motion of the diaphragm can be materially increased, thereby providing a loudspeaker having high fidelity and non-directional property. The inside and outside voice coils of the diaphragm and the flux density in the corresponding gaps of the magnetic circuit can be so selected that the diaphragm may vibrate under best and balanced state. Thus a loudspeaker can provide good tone with minimum distortion. The inputs to the inside and outside voice coils can be adjusted so that best characteristic may be obtained. That is, unbalance in operation of the inner and the outer support portions of the diaphragm can be controlled so that the diaphragm may produce perfect piston motion. Such a control cannot be performed in the conventional annular-diaphragm loudspeaker. A horn speaker has such advantageous properties as large vibrating area, high rigidity, low mass and increased driving force, so that radiation efficiency is high and substantially flat characteristic is obtained in the higher frequency range. The horn loudspeaker, having large vibrating area, light weight and rigid construction, is particularly suitable to a loudspeaker having a short horn, wherein the size or length of the horn can be made substantially smaller or shorter.
This compression driver has an improved phasing plug. The improved impedance match provided by the phasing plug allows more acoustic power to be transferred from the diaphragm, particularly at low frequencies. The phasing plug reduces the apparent size of the annular diaphragm, thus improving high frequency response and dispersion. In most applications, the throat diameter at the horn is small compared to the diameter of the annular diaphragm. The phasing plugs for use with compression drivers driven by an annular or ring diaphragm have consisted of a plug having an annular slot located next to, and concentric with, the annular ring diaphragm. The phase plug contained an annular, axially symmetric passageway connecting the annular slot to the mouth of the horn. The annular passageway typically expanded in cross section from the diaphragm to the throat so as to nearly cover the entire throat of the horn. However, the phasing plug utilizing an annular slot adjacent to the diaphragm exhibits poor dispersion characteristics at higher frequencies because the apparent size of the source is large compared to the wavelength.
U.S. Pat. No. 5,537,481 teaches a horn driver which includes a driver body and pole piece positioned therein. A throat extends through the pole piece along a longitudinal axis through the horn driver. A magnet assembly, attached to the driver body, is positioned above the upper portion of the pole piece and spaced therefrom to define a diaphragm chamber. A disk-shaped diaphragm is placed above the diaphragm chamber and is spaced from the pole piece and below and spaced from the magnet assembly. The diaphragm is attached to the magnet assembly solely at a central support area. The diaphragm has a ring-like and vibratable portion extending radially outward from the central support area to an outer peripheral edge and a voice coil connected to a cylindrical voice coil support along the outer peripheral edge of the diaphragm. The portion of the diaphragm includes an inner diaphragm segment extending upwardly and outwardly from the central support area to a peak point and an outer diaphragm segment extending downwardly and outwardly from the peak point to the outer peripheral edge. The upper portion of the pole piece has an upper surface shaped similar to and following the diaphragm portion. The spacing between the diaphragm portion and the pole piece increases continuously in a non-linear manner from a minimum near the peripheral edge to a maximum near the central support area. The horn driver includes a device for generating a magnetic field passing through the voice coil and electrical connections to the voice coil.
U.S. Pat. No. 4,325,456 teaches a phasing plug as an acoustic transformer. The phasing plug has the general shape of a doubly truncated cone with an annular surface located on the larger end of the truncated cone and positioned adjacent to the diaphragm. The conical surface of the cone has spaced radial slots or channels formed therein connecting the truncated surfaces of the cone. These channels form air passageways for propagation of sound waves. The walls of the slots or channels are tapered such that the cross-sectional areas of the channels increase from their inlet ends near the speaker diaphragm, towards the outlet ends, positioned at the throat of the horn. The phasing plug provides a mechanical impedance match between the output of the annular diaphragm and the input of the horn.
Traditionally, the compression drivers are limited to use with either convex or concave-domed, diaphragms. While spherical shell diaphragms are suitable for use in high frequency loudspeakers, it has been found that such diaphragms are typically inappropriate for use with mid-range frequency loudspeakers. For example, a typical mid-range driver requires a 50 to 70 square inch diaphragm surface in order to generate appropriate frequency signals. Since spherical shell diaphragms are vibrated by means of voice coils around the perimeter thereof, a mid-range driver incorporating such a spherical shell diaphragm would require an inordinately large voice coil. The cost and weight of a magnet structure driving the voice coil is generally deemed to be prohibitive.
A compression driver includes a pole piece made of ferromagnetic material which has a bore therein, the front end or opening of which is adaptable for coupling to the throat of a horn. A diaphragm, usually circular with a central dome-shaped portion, is mounted adjacent the rear opening of the bore so as to be freely vibratable. Attached to the edge of the dome of the diaphragm is a cylindrical coil of wire, the voice coil, oriented so that the cylindrical axis of the coil is perpendicular to the diaphragm and coincident with the axis of the pole piece bore. A static magnetic field, usually produced by a permanent magnet, is applied so that an alternating current flowing through the voice coil causes it to vibrate along its cylindrical axis. This in turn causes the diaphragm to vibrate along the axis of the bore and generate sound waves corresponding to the signal current. The sound waves are directed through the bore toward its front opening. The front opening of the bore is usually coupled to the throat of a horn, which then radiates the sound waves into the air. In the description that follows, the term "throat" is used to mean either the front or downstream end of the pole piece bore or the actual throat of a horn. Interposed between the diaphragm and the pole piece bore is a perforated phasing plug. Within the phasing plug are one or more air passages or channels for transmission of the sound waves. The surface of the phasing plug opposite the diaphragm is of corresponding sphericity and positioned fairly close to the diaphragm while still leaving an air gap, or compression region, in which the diaphragm can vibrate freely.
In order to provide a low reluctance magnetic pathway for the applied static magnetic field, the permanent magnet and the voice coil are disposed within a surrounding environment of ferromagnetic material. As both the magnet and voice coil are commonly located on the side of the diaphragm facing the pole piece, the magnetic pathway includes both the phasing plug and the surrounding pole piece. In order for the voice coil to be free to vibrate, however, it must be disposed within an annular air gap, which will be referred to herein as the coil space. Ideally, the coil space should be made as small as possible since air in the magnetic pathway adds reluctance to the magnetic circuit which lessens the field strength at the voice coil. Nevertheless there is a considerable volume of air in the coil space surrounding the voice coil as well as in the spaces along the inner edge of the surround and outer edge of the diaphragm, which are continuous with the coil space. This region, including the coil space and the space along the surround and outer edge of the diaphragm, is thus an uncoupled region since it is so far from the inlets of the phasing plug air passages that variations of air pressure in that region are coupled little or not at all to the phasing plug and thence to the throat. These pressure variations thus result in energy losses that lead to heating of the loudspeaker but do not result in the generation of useful sound output. The uncoupled region also causes cavity resonance effects that distort the overall sound output of the speaker due to anomalies in its frequency response. Such resonances, known as parasitic resonances, present a significant design problem for the speaker designer ("The Influence of Parasitic Resonances on Compression Driver Loudspeaker Performance" by Kinoshita, et al. presented at the 61st Convention of the Audio Engineering Society in 1978 and available as preprint no. 1422 (M-2).). It would be useful to couple the pressure variations in the uncoupled region around the voice coil to the throat of the horn, in addition to the pressure variations produced by the diaphragm, to improve the efficiency and sound quality of the loudspeaker. Use of the additional pressure variations could be expected to reduce heating in the region around the voice coil as a result of repeated compression and rarefaction of the same air in that region, to produce an increase in the efficiency of the loudspeaker, and to reduce parasitic resonances.